In humans, congenital cerebellar ataxias are fairly common birth defects of the brain that cause significant morbidity and mortality in pediatric populations. Despite their clinical prevalance, the genetic and developmental basis of these diseases are poorly understood. Extensive work in model organisms has demonstrated that Purkinje cells, with their elaborate, extensively branched, stereotypical dendritic arbors are essential to normal cerebellar function. Furthermore, Purkinje cell function is directly related to its cell morphology. However, very little is known about the intrinsic factors and mechanisms driving Purkinje cell morphological development. The spontaneous neurological mouse mutant, tippy, holds great potential to provide insight into the intrinsic Purkinje cell dendritic developmental program. Preliminary phenotypic analysis demonstrates that Purkinje cells of tippy mutant mice have defects in dendritogenesis and spinogenesis that are most likely not a consequence of extrinsic factors. In this proposal, a positional cloning strategy will be used to identify the molecular lesion in tippy mutant mice and to identify the causative tippy gene (Aim I). To obtain a detailed understanding of the cerebellar phenotype, a number of quantitative and qualitative immunohistochemical analyses will be performed (Aims II and III). Specifically, the tippy mutant Purkinje cell branching and spine defect will be quantified using camera lucida drawings of Golgi-stained Purkinje cells in Aim II. Various immunohistochemical and anterograde labeling experiments will be performed in Aim III to more fully characterize the distribution and composition of tippy Purkinje cell synapses with their afferents. Once the tippy gene is identified, the mechanism by which it causes the Purkinje cell phenotype will be investigated with the broader aim of using the tippy mouse as a model to dissect out the intrinsic developmental signals that regulate Purkinje cell dendritic development. The much needed illumination of the intrinsic Purkinje cell dendritic developmental program will likely provide a more comprehensive understanding of the molecular and biochemical basis of numerous congenital ataxias. [unreadable] [unreadable]